Black Carbon Contribution to Organic Carbon Stocks in Urban Soil

Soil holds 75% of
the total organic carbon (TOC) stock in terrestrial
ecosystems. This comprises ecosystem-derived organic carbon (OC) and
black carbon (BC), a recalcitrant product of the incomplete combustion
of fossil fuels and biomass. Urban topsoils are often enriched in
BC from historical emissions of soot and have high TOC concentrations,
but the contribution of BC to TOC throughout the urban soil profile,
at a regional scale is unknown. We sampled 55 urban soil profiles
across the North East of England, a region with a history of coal
burning and heavy industry. Through combined elemental and thermogravimetic
analyses, we found very large total soil OC stocks (31–65 kg
m–2 to 1 m), exceeding typical values reported for
UK woodland soils. BC contributed 28–39% of the TOC stocks,
up to 23 kg C m–2 to 1 m, and was affected by soil
texture. The proportional contribution of the BC-rich fraction to
TOC increased with soil depth, and was enriched in topsoil under trees
when compared to grassland. Our findings establish the importance
of urban ecosystems in storing large amounts of OC in soils and that
these soils also capture a large proportion of BC particulates emitted
within urban areas.